1. Field of the Invention
The present invention relates to image processing methods and apparatuses, and more particularly, to a method for processing digital image signals.
2. Description of the Related Art
Thin display devices, liquid crystal displays (LCDs), plasma displays (PDPs), and field emission displays (FEDs) have attracted attention.
The LCDs, PDPs, and FEDs are fixed-pixel matrix-driven display devices, which can be driven by digital image signals. The number of grayscale levels of the above-described display devices is represented by the number of bits of a video signal corresponding to each pixel.
Techniques for displaying images so that they can be visually aesthetic to the human eye by performing signal processing on image signals are being considered. Such techniques include edge enhancement processing for enhancing edge portions and high-frequency components of images so as to apparently increase the resolution of the images.
FIG. 8 illustrates the configuration of an edge enhancer 800 for performing edge enhancement processing on image signals.
An image signal input from an input terminal 801 is output to a high-pass filter 803 and also to an adder 807.
The high-pass filter 803 extracts high-frequency components of the input image signal and outputs the resulting image signal to a multiplier 805.
Under the control of a controller 809, the multiplier 805 multiplies an enhancement coefficient, indicating the level of enhancement of the high-frequency components of the image, by the high-frequency components, and outputs the resulting signal to the adder 807. By controlling the enhancement coefficient, the level of enhancement of the high-frequency components of the image can be adjusted.
As the bit precision of the high-frequency components which are output from the multiplier 805, an 8-bit image signal input from the input terminal 801 can be increased to a 12-bit image signal by the high-pass filter 803 and by the multiplier 805.
Then, the adder 807 adds the original 8-bit image signal and the 12-bit high-frequency components output from the multiplier 805 and outputs the resulting 12-bit image signal having enhanced high-frequency components to a rounding unit 811.
By outputting an enhancement coefficient having a negative sign from the controller 809, the image can be made smoother instead of enhancing the edges.
To convert the 12-bit image signal into an 8-bit image signal, the rounding unit 811 truncates the lower four bits of the 12-bit image signal by a rounding operation, and outputs the resulting 8-bit image signal to an output terminal 813.
In digital broadcasting, which has recently started, the resolution of broadcasting programs varies, such as 720 pixels in the horizontal direction and 480 pixels in the vertical direction, which are equivalent to those in current terrestrial broadcasting, and 1920 pixels in the horizontal direction and 1080 pixels in the vertical direction, which are used in high-definition broadcasting.
It is thus necessary to perform resolution conversion processing on various types of input image signals to the resolution of a display device.
FIG. 9 is a block diagram illustrating the configuration of an image processing apparatus for performing resolution conversion on an image signal.
An image signal input from an input terminal 901 is supplied to a resolution converter 904.
The resolution converter 904 converts the resolution of the input signal to the resolution of a display device (not shown) under the control of a controller 909.
When the input image signal has 8 bits, the resolution converter 904 expands the 8-bit image signal into a 12-bit image signal to maintain the precision of the converted image signal. The 12-bit image signal is then output to a rounding unit 911.
The rounding unit 911 reduces the input 12-bit image signal into 8-bit data by truncating the lower four bits, and outputs the 8-bit data to an output terminal 913.
The controller 909 controls enlargement/reduction processing in the resolution converter 904.
In printers, halftone processing using dithering processing has been performed as a binarizing method. In printers, such as that disclosed in Japanese Patent Laid-Open No. 2000-134471, images are divided into, for example, a character portion and a photograph portion, and different binarizing methods are used for these portions. Japanese Patent Laid-Open No. 6-178087 discloses a method for enlarging and reducing binary images.
Japanese Patent Laid-Open No. 2003-69830 discloses an image processing method for performing resolution conversion and dithering processing.
If, after the number of bits of a pixel signal is increased by performing resolution conversion, the number of bits of the pixel signal is reduced simply by performing the rounding operation, pseudo contours may easily occur depending on the type of image.
In particular, as in natural images, for example, a blue sky, in images having a narrow dynamic range, the correlation of adjacent pixels is high, and pseudo contours easily occur, which is visually noticeable.
To prevent the occurrence of pseudo contours, dithering processing can always be performed instead of the rounding operation. In this case, however, dithering processing does not produce a noticeable effect on images having a wide dynamic range or images subjected to edge enhancement, since pseudo contours do not easily occur because of the low correlation between adjacent pixels of such images. Conversely, dithering processing easily produces an adverse influence, for example, noise having a fixed pattern, which is noticeable.